The ability to store information is an essential property for many applications of organic electronics. RFID chips, for example, have to be able to receive and emit stored information, which is communicated by radio signal. In point of fact, the storage of information is based on the use of memory devices, in which advantage is taken of the hysteretic behavior of a physical property of a material in response to an applied electric field. The information stored is read by measuring the physical property in question.
A disadvantage of conventional means for the storage of information, such as capacitors, is the tendency of the latter to run down over (the term used is volatility of the memory); it is thus necessary to restore the information stored at regular time intervals, which is not possible for devices, such as RFID chips, which do not have available a permanent energy source.
Consequently, it is important to have available nonvolatile and rewritable memory devices. One of the possible routes for achieving this is to resort to a ferroelectric polymer. This is because the latter exhibits a zero-field remanent polymerization which it is possible to reverse by the application of an appropriate voltage.
Many authors have provided for the use of the polyvinylidene fluoride/trifluoroethylene copolymer, or P(VDF-TrFE), for this type of application.
Thus, the document WO 02/43071 describes ferroelectric memory circuits based on the use of ferroelectric polymers of the polyvinylidene fluoride family and in particular P(VDF-TrFE). The document plans to make available a contact layer comprising a conducting polymer, in contact with the layer of ferroelectric polymer, in order to improve the performance of the circuits.
The document High-performance solution-processed polymer ferroelectric field-effect transistors by Naber et al. in Nature Materials, 4 243-248 (2005), describes the use of a P(VDF-TrFE) polymer as gate insulator, in combination with poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene-vinylene] as semiconducting material, for the preparation of a ferroelectric field-effect transistor.
The document Printed non-volatile memory for a sheet-type communication system by Sekitani et al. in IEEE Transactions on Electron Devices, 56, 1027-1035 (2009), describes the use of printing technologies for the manufacture of nonvolatile memory devices with a ferroelectric ink based on P(VDF-TrFE).
The document Organic nonvolatile memory devices based on ferroelectricity by Naber et al. in Adv. Mater., 22, 933-945 (2010), is a review of the different types of nonvolatile memory devices using a ferroelectric polymer. Only PVDF and P(VDF-TrFE) are mentioned as suitable ferroelectric polymers.
The document Recent advances in ferroelectric polymer thin films for memory applications by Furukawa et al. in Current Applied Physics, 10, e62-e67 (2010), is a review of the properties of P(VDF-TrFE) copolymers in the context of their applications in memory devices.
The document Compression of cross-linked poly(vinylidene fluoride-co-trifluoroethylene) films for facile ferroelectric polarization by Shin et al. in ACS Appl. Mater. Interfaces, 3, 4736-4743 (2011), describes a process for crosslinking P(VDF-TrFE) and is concerned with the effects of the pressure on the ferroelectric properties of the polymer.
The document Organic ferroelectric memory devices with inkjet-printed polymer electrodes on flexible substrates, by Bhansali et al. in Microelectronic Engineering, 105, 68-73 (2013), describes the manufacture of a matrix ferroelectric memory device by printing semiconducting electrodes made of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate on a P(VDF-TrFE) layer.
However, P(VDF-TrFE) copolymers have the disadvantage of exhibiting a high coercitive field strength (of more than 50 V/μm). The coercitive field strength is the threshold electric field value which makes possible the cooperative switching of the dipoles of the material and thus the inversion of its polarization. When the coercitive field strength is high, it is necessary to apply high voltages to the memory devices, which results in an excessive consumption of energy, risks of electrical breakdown of the material and also the need to use very thin layers of ferroelectric material.
The document P(VDF-TrFE-CFE) terpolymer thin-film for high performance nonvolatile memory by Chen et al. in Applied Physics Letters, 102, 063103 (2013), suggests the use of a terpolymer comprising a molar proportion of 60.3% of VDF, 32.6% of TrFE and 7.1% of chlorofluoroethylene for the manufacture of ferroelectric memory devices.
This terpolymer, which belongs to the category of “relaxer” polymers, exhibits a lower coercitive field strength than P(VDF-TrFE) copolymers, of the order of 20 V/μm. On the other hand, the document does not specify that the remanent polarization of the terpolymer is low, which makes it difficult to use it for nonvolatile memories, and neither does it specify that its Curie temperature is low and close to ambient temperature. In point of fact, above the Curie temperature, the material loses is ferroelectric properties. The use of this polymer in memories thus cannot be envisaged in practice.
Thus, there exists a need to develop ferroelectric memory devices simultaneously exhibiting a relatively low coercitive field strength, a relatively high remanent polarization and a relatively high Curie temperature.